Can not receive Email.. Need Help

Hi all,
Earlier this morning, my exchange server 2007 (hosted on SBS 2008) just
stopped receiving external e-mail. We can send interoffice e-mails and we can
send out e-mails to other locations fine, but we are unable to receive any
e-mail. All internet connections are normal as well, and the port 25 is open
on the firewall.

The error from when we try to send an e-mail to the server is this:
“This is an automatically generated Delivery Status Notification. Delivery
to the following recipients failed.”

Earlier today I installed the anti-spam update from MS “Microsoft exchange
server standard filter update V3.39223.513” and reboot the server.
Does anybody have similar issues?
-Eric